This invention relates to compensation apparatus for a piezoresistive sensor employing the shear stress effect.
The prior art has been cognizant of the fact that a single resistive device could be adequately employed as a pressure sensor. In regard to such operation it was determined that if a longitudinal current is caused to flow through a cross-shaped element then a transverse voltage is generated in such a piezoresistive device subjected to pressure. This voltage can be detected via voltage contacts located at opposite edges of the piezoresistor. As indicated, the effect is well known and has been widely described in technical literature and shown in various patents. Thus, it was determined that a complete measurement of the biaxial stress or shear stress can be obtained using a simple gage of a cross or X shaped configuration. This structure indicates a single resistor of such a configuration deposited on a diaphragm and which resistor will provide a voltage output proportional to a pressure applied to the diaphragm. The concept and structure has been depicted in a paper entitled Semiconductor Stress Transducers Using Transverse and Shear Piezoresistance, by W. G. Pfann and R. W. Thurston, Bell Telephone System Technical Publication (Monograph 4056) (1961). In this publication there is shown on Page 3, FIG. 2 a semi-conductor gage for the measurement of strain or biaxial stress.
The device as well as the theory of operation is further explained in detail in U.S. Pat. No. 3,123,788 entitled Piezoresistive Gage issued on Mar. 3, 1964 to W. G. Pfann et al. As one can see from the structure, the device depicted is essentially a single resistance of a cross-like shape secured to a suitable diaphragm. The device is biased via an excitation voltage applied between the vertical arms this voltage generating a longitudinal current and a voltage indicative of applied pressure is derived across the horizontal arms or terminals. A further example of such a device is depicted in U.S. Pat. No. 4,317,126 entitled Silicon Pressure Sensor which was issued on Feb. 23, 1982 to J. E. Graff, Jr. The operation and structure of this device is essentially identical to those above described.
In any event such devices tend to exhibit good balance and temperature operation if they are completely symmetrical in all respects. However, this aspect is beyond the capability of modern day technology. Therefore, in spite of such representations by the prior art, the so-called shear gage exhibits undesired offsets which are temperature sensitive and which considerably affect the operation of such pressure sensors.
It is therefore an object of the present invention to provide improved apparatus for compensating and normalizing the output of a shear gage configuration as above described.